1. Field of the Invention
The present invention relates to plastic light conduits, or fiber optics, and a method and apparatus for making them. More specifically, the present invention relates to a high temperature plastic light conduit, having good light transmitting characteristics, low light leakage, few, if any flaws, that does not melt, even above 150.degree. C., to about 350.degree. C., and which remains flexible indefinitely. The present invention also relates to a method and apparatus for making such a light conduit.
2. Description of the Prior Art
Increasingly, light conduits are used in a wide variety of applications. Glass-fiber light conduits are commonly used in communications, and for low level illumination purposes over short distances. Efforts to use such glass-fiber light conduits for general or specialized illumination have been largely unsuccessful because such fibers are expensive, small in diameter, brittle, heavy, and will not transmit large quantities of light under ordinary conditions. The amount of light transmitted by glass-fiber light conduits can be increased by bundling a large number of glass fibers into a single light conduit, allowing the light conduit to transmit a greater quantity of light. Such bundles are, however, expensive to manufacture and the adhesives used to bind the individual glass fibers together break down in the presence of oxygen at temperatures of approximately 250.degree. C., to about 350.degree. C., which are commonly produced by ordinary incandescent lamps. In addition, even such bundles are limited to a maximum size of approximately one-half inch to three-quarters inch in diameter. Furthermore, a light conduit for general illumination does not require the fine properties of glass-fiber light conduits.
Many efforts to provide an inexpensive, more flexible, and durable light conduit for general illumination have been made. These efforts led to plastic light conduits, usually consisting of thermoplastic polymers. Most conventional plastic optical fibers comprise a core of poly(styrene) or poly(methylmethacrylate), which cannot withstand temperatures above about 80.degree. C. At higher temperatures of about 80.degree. C., these light conduits shrink, drawing the tip of the fiber away from the focal point of the lamp and reducing the amount of light transmitted, and then melt into a viscous liquid, which terminates the light transmitting capabilities of the light conduit. Moreover, in the presence of oxygen at temperatures of about 120.degree. C., such light conduits become brittle and discolored, typically turning amber or brown, which discolors and severely attenuates the light they transmit.
One effort to overcome these difficulties is explored in U.S. Pat. No. 4,505,543, which advances the prior art by adding a quantity of styrene derivative selected from the group consisting of p-phenylstyrene, p-tert-butylstyrene, p-cyanostyrene, p-iodostyrene, a,b,b-trifluorostyrene, perfluorostyrene, and 2,3,4,5,6-pentafluorostyrene to the conventional recipe, thereby increasing the temperature at which shrinkage begins to more than about 100.degree. C. Like the prior art from which it is derived, however, U.S. Pat. No. 4,505,543 is a thermoplastic light conduit, produced through bulk polymerization to a polymerization degree of substantially 100%, which is subsequently re-melted and then spun into a light conduit through a ram extruder.
Another example of a thermoplastic light conduit is set forth in U.S. Pat. No. 3,993,834, which discloses a methyl methacrylate polymer core, produced by continuous bulk polymerization, and subsequent removal of volatile contents, using alkyl acrylates to co-polymerize with methyl methacrylate. A free radical initiator, such as butyl peroxide is added to the co-polymer, typically comprising methyl methacrylate at 85% by weight, and up to 20% by weight of units derived from at least one alkyl acrylate and/or alkyl methacrylate other than methyl methacrylate, to produce a polymer, which is stripped of volatile contents through conventional means, and then is extruded through an appropriate dye, while molten, to be formed into desired shapes such as pellets. The product of this process is then melted and extruded through a spinneret to produce a light conduit. The resulting product has a degree of polymerization of about 65%. Obviously, the product must be thermoplastic, and therefore subject to the difficulties encountered with ordinary thermoplastic light conduits.
Another thermoplastic light conduit is disclosed in U.S. Pat. No. 3,930,103, which employs a core material of poly(methyl methacrylate) resin and a sheath of vinylidene fluoride polymer. Like other light conduits of this type, it must be stretched to impart bending strength. In this particular case, the light conduits are stretched to 1.5 times their original length at 140.degree. C., thereby thinning the light conduit and significantly decreasing the proportion of light transmitted through it. Such light conduits are also thermoplastic, and are formed by extrusion processes.
Other examples of thermoplastic organic polymer light conduits include U.S. Pat. No. 3,900,453 and U.S. Pat. No. 3,252,950. Such light conduits are ordinarily extruded, which requires that they be thermoplastic light conduits. All of them are linear organic polymers, made from vinyl-type monomers and certain other ingredients, designed to stimulate and control polymerization.
All thermoplastic polymers capable of transmitting light, however, melt rapidly when exposed to heat in excess of about 120.degree. C., at the most, after shrinking. Accordingly, they are wholly unsuitable for any application involving exposure to such temperatures. Although probably most desired applications of light conduits require them to operate in temperatures of less than 120.degree. C., light must be transmitted into the light conduit efficiently in order to transmit quantities of light required for many uses, and it is generally not possible to transmit significant amounts of light into a light conduit without also exposing the light conduit to temperatures well in excess of 120.degree. C. Therefore, use of thermoplastic light conduits seriously limits the amount of light that can be shown into the light conduit, diminishing its utility.
In addition, plastic light conduits according to the prior art often have defects including voids, bubbles and trapped impurities. Such defects significantly diminish the ability of the light conduit to transmit light because they scatter and absorb the light, and may alter the transmitted light color temperature.
Voids in a light conduit create two media interfaces: conduit-to-air; and air-to-conduit. Highly significant amounts of light are lost at each interface. Some of the light is reflected back toward the light source and some is scattered out the sidewall of the light conduit. Bubbles cause the same effects. Although the effect of a single bubble is small, very often a prior art conduit is so filled with bubbles that much of the light is lost.
This prior art all involves thermoplastic polymers, which are linear chain-linked polymers. A polymer is a large molecule built up by the repetition of small simple chemical units, known as monomers. The repetition of the monomer may be linear, much as a chain is linear, or branched with the same morphology as linear polymers (both producing thermoplastic materials), or it may be interconnected or cross-linked to form three-dimensional networks of the repeat unit (which forms thermoset materials).
These two different classes of polymers, also recognized as thermoplastic and thermoset polymers, respectively, exhibit quite different physical properties. For example, crude rubber consists of long linear chains of isoprene molecules, typically upwards of 20,000 units long, which is very stretchable and very sensitive to heat. Vulcanization, typically performed with the aid of sulfur, converts the linear polymer chains into a network polymer of three dimensions, which greatly increases hardness and elasticity. In the same way, a cross-linked or thermoset organic polymer light conduit would have physical properties greatly superior to linear or thermoplastic polymer light conduits.
Thermoplastic polymers become soft when heated and hard when cooled. They usually melt at temperatures above about 80.degree. C. to about 200.degree. C. Additionally, a large number of thermoplastics tend to either oxidize and consequently deteriorate or de-polymerize and/or decompose to oligomers and/or monomers. Conversely, thermoset polymers become harder when exposed to increasing temperatures. They do not melt when exposed to temperatures in the 150.degree. range. They strongly resist deterioration caused by heat in the absence of oxygen.
Therefore, a significant need exists for a light conduit that is lightweight, flexible, will not deteriorate with age, even though exposed to heat in excess of about 120.degree. C. to about 350.degree. C. at the tip that is exposed to the light and about 250.degree. C. throughout its length, has good resistance to the surrounding environment, has good light transmission characteristics, does not shrink substantially, does not melt, oxidize or otherwise deteriorate when exposed to high temperatures, remains flexible, and is relatively easy and inexpensive to produce.